Two men were fighting for life today after six volunteers fell seriously ill while taking part in a trial of a new drug.

The six men were admitted to the intensive care unit at Northwick Park hospital, north-west London, from an independent medical research unit on the hospital campus after reacting badly to the drug, which is to treat chronic inflammatory conditions and leukaemia.

It was reported that one man's head had swollen up to three times its normal size.

One of the victims was named as student Ryan Flanagan, 21, of Highbury, London. A national newspaper said he was taken to intensive care three hours after taking the tablets.

His family were told he could not breathe and his head and neck had swollen to three times normal size.

Family friend Sarah Brown, 27, told the paper: "Ryan was a healthy young man and he saw the trial advertised on the internet.

"He is at college and was doing it to make a bit of extra money."

A hospital spokeswoman said today that the six remained in intensive care. Two of them are in a "critical condition", she said. The other four are "serious but stable".

Parexel, the company running the trial, said it had operated within regulatory guidelines and such adverse reactions were extremely rare.

Another national paper reported the drug is known as TGN1412, and is made by the firm TeGenero AG, based in Wurzburg, Germany.

The men fell ill on Monday evening. The UK medicines watchdog, the Medicines and Healthcare products Regulatory Agency (MHRA), immediately suspended the trial and sent inspectors to the unit to investigate.They are working with health authority chiefs, the Department of Health and the police, to find out what happened.

Researchers running the trial have assured the MHRA that nobody other than the six men affected by the drugs has been given the product.